1. Field of Invention
The present invention generally relates to automatic high-speed labeling machines and a method they may utilize to transfer labels onto containers without depositing a glue residue on the containers.
More particularly, the present invention pertains to a method or process for transferring labels to containers by utilizing the surface tension of water, rather than by glue as is customarily done, and thereby eliminating any glue residue on the surface of said container.
Even more specifically, the present invention concerns a method for applying a thin uniform coating of water, with or without a surfactant, onto the surface of containers in high-speed labeling machines so that tangential contact between a leading edge of each pre-cut label on a label transport means will cause said label to transfer to the surface of said container by surface tension of water alone and thereby cause said label to be lifted from said transport means, wrapped around said container and subsequently glued to itself rather than to the container, but being held thereon by friction after evaporation of the water.
2. Discussion of the Prior Art
It has been estimated that Americans generate more than one billion pounds of plastic waste annually. More than 870 million pounds are from plastic soft drink containers and, although these bottles are recyclable, 80 percent are estimated to be buried in landfills.
Virtually all plastic soda bottles and bottles for many other products such as mouthwash, liquor, cooking oil, juice, etc., are made from polyethylene terephthalate or PET. The typical clear plastic PET bottles also has a cap made from aluminum or polyethylene and in many cases a label is connected to the PET bottle with ethylene vinyl acetate (EVA) or other hot-melt glues. Therein lies the problem for recyclers.
In common recycling processes the whole plastic bottle is ground, sent through a stream of air to separate dirt and fine particles, washed vigorously and then separated into several streams of useful products including PET granules, polyethylene granules and aluminum chips, shredded paper and other contaminants such as glue. Recovered PET granules can be resold for moulding new, clear plastic bottles provided there are no contaminates present, such as glue which can discolor the appearance and limit its use for other applications where appearance does not matter.
Because glue residue on plastic containers represents one of the greatest obstacles to rapid recycling, there is a demonstrative need for a method adaptable to conventional labeling operations which produces a container surface free of glue residue.
In this regard, several inventions in the prior art have addressed this problem and a few have also employed water in an adhesive process, but none have utilized the surface tension of water in labeling operations.
For example, in U.S. Pat. No. 4,574,020 (1986), Fosnaught discloses an apparatus and method for wrapping a plastic label around a container without leaving a glue residue thereon. His method employs a solvent, such as methylene chloride, which when applied to a leading edge of a polymer backing on a film label produces an instant but temporary tacky liquid, but only momentarily while in tangential contact with a container, thereby providing the adhesive necessary to lift said label from a transport means onto a container and thereby further allowing said container to literally wrap the label around itself. Since the film labels of Fosnaught's invention contain a layer of foam polystyrene, the trailing edge, being of sufficient length to overlap the leading edge, is also treated with the solvent in order to bond the label unto itself.
Although Fosnaught's patent also speaks of a liquid recovery system for the solvent, it does not address containment nor recovery of solvent vapors, which by necessity of his invention must evaporate. Thus, it can be easily seen that in a high-speed labeling operation, say 400 containers per minute, a substantial amount of volatile solvent will be released into the atmosphere, and perhaps even into the containers themselves, thereby posing very serious environmental and health concerns, especially with methylene chloride.
By contrast, the present invention employs water either by itself or in conjunction with very slight traces of surfactant, such as ammonia, to wet the entire surface area of a container prior to contacting a leading edge of a label so that the surface tension of water alone is sufficient to lift and remove said label from an ordinary transport means and wrap it around the container. Glue still needs to be applied to a label's trailing edge which overlaps it leading edge for a permanent adhesive bond: however the water simply evaporates. Therefore, it may be easily seen that one major advantage of the present method is that it can be adapted to a variety of present labeling machines by simple substitution of a water application means, such as a water wheel, in place of a primary glue application means while maintaining any secondary gluing means for a label's trailing edge.
There are also a few other methods in the prior art that employ water as an intermediary step in bonding processes which should be further distinguished from the present invention.
In U.S. Pat. No. 3,869,329 (1975), Schweltzer, Jr. and Schupak disclose a method of sealing nylon film using boiling water or steam to bond together surfaces of very thin nylon film without the application of pressure, wherein the surfaces of said film are exposed to the water as the only adhering solvent. Although temperature is known to affect surface tension, the present invention does not require elevated temperatures of water to cause a label transfer onto common containers because such transfer is easily accomplished at room temperature wherein most labeling operations occur, however the present method may also require employment of elevated temperatures of water in labeling operations where wettability need be enhanced between unusual materials. Moreover, the Schweltzer invention should be further distinguished as providing a bonding together of like surfaces, while the present invention is particularly applicable to unlike surfaces, such as polyethylene film labels to polyester containers, or, for that matter, paper onto glass.
In U.S. Pat. No. 4,605,454 (1986), Sayovitz, et. al., describes a method of ultrasonically bonding non-woven webs by employing water to form web laminates, such as plastic polymeric webs for tents and the like. His patent is mentioned because of its spray application of a generally uniform coating of water prior to contacting component layers for bonding, while further recognizing that other liquids, such as alcohols or mixtures of alcohol and water may very well need be used depending on the nature of materials being laminated.
The same principle may be true in labeling operations, and because of the diverse nature of materials that undergo labeling, surface tension of water may very well need to be slightly adjusted to enhance wettability in some situations. Therefore, such surface tension enhancement by surfactants and temperature elevation may be employed in labeling machines generally used by those skilled in the art to label a variety of containers and, accordingly, should be considered within the scope and spirit of our method.
For example, it has long been known that the ability of a liquid to wet a solid surface depends largely on the surface tension of the liquid which is often defined as the force acting in the surface of a liquid tending to minimize to area of the surface produced by internal forces within the liquid. Most liquids have surface tensions of 20-40 dynes/cm, but water has the exceptionally high value of 72.75 dynes/cm at 20 degrees centigrade due to its molecular polarity or internal cohesive forces which also affects its wettability on various surfaces.
Because wettability of a surface is also a reason for label transfer to a surface of a container, it is important to understand that the wetting or non-wetting ability of a liquid depends on the chemical composition of the surface and is often shown and expressed by a contact angle between a solid surface and a droplet of liquid, as measured through the droplet, where the liquid is said to wet a solid if the contact angle lies between 0 and 90 degrees and not wet the solid when the contact angle lies between 90 and 180 degrees. In other words, the more a liquid spreads out on a surface the more it is said to wet that surface. This phenomenon is easily seen in the waterproofing of surfaces which would otherwise be pervious to water but by proper application of a surface film causes water droplets to form beads with a contact angle of more than 90 degrees.
When a surfactant, such as a detergent, is added into a liquid the surface tension of the liquid is generally lowered thereby increasing its wettability, and since surfactants are mainly composed of molecules containing both a polar (hydrophilic) and non-polar (hydrophobic) group, the result is orientation of the greater concentration of surfactant in the interface phase rather than in the bulk of the liquid. Moreover, surfactants are commonly classified as anionic, cationic and non-ionic. In anionic surfactants, such as detergents, the hydrophilic group is a polar negatively charged ion like carboxylate, sulfate or phosphate. In cationic surfactants the hydrophilic group is most often quaternary nitrogen and in non-ionic surfactants attraction between one end of the molecule and the polar liquid is caused by hydrogen bonding such as with alcohols.
The foregoing discussion is relevant to the mechanics of label transfer to a container utilizing water in automatic high-speed labeling machines because not all containers not labels are made from the same solid materials. For example, PET containers are polyesters which contain polar groups while their popular film labels are plastic polyethyenes which contain non-polar groups. Therefore, the surface tension of water utilized to transfer a label onto a container may very well need to be adjusted by a surfactant so that wettability by the water is sufficient enough to wet both surfaces, but yet at the same time maintain the maximum internal cohesive forces of the liquid so that it will not pull apart during high-speed labeling operations.
The utilization of water is particularly well suited for labeling cylindrical containers in so-called "straight-through" labeling machines, but of course may also be easily adopted by other types of those machines where there is a need to eliminate glue residues on a container's surface. An example follows.
In our recent U.S. Pat. No. 5,082,520 (1992) entitled "Automatic High-Speed Labeling Machine Employing Various Linear and Rotational Speeds of The Container," we disclosed a labeling machine which propels containers linearly and upright but while also causing them to rapidly rotate counterclockwise around their vertical axes by a set of parallel upper and lower bottle spinning belts traveling clockwise but in slightly slower partial parallel cooperation with a container drive belt to place the container in brief tangential contact with a label transport wheel. It is this type of a machine that is particularly well suited by only slight modification for the practice of our method because the machine is of the type which employs a container drive belt to laterally contact and propel said containers both lineraly and rotationally past a label transfer means for label transfer. Thus, these machines are easily modified by installation of a water application means preferably in combination with a container drive belt so that a uniform film of water coating is applied to each container prior to contacting a label. Furthermore, machines of this type can be also easily adjusted for proper placement of glue on a trailing, rather than a leading edge, of a label.
Accordingly, it is a general object of the present invention to provide a method of utilizing water to transfer labels onto containers in automatic high-speed labeling machines, thereby eliminating glue residues on the containers.
It is another object of the present invention to provide a method and apparatus whereby a thin film of water, either by itself or containing surfactants, uniformly covers the surface of a container to produce the requisite surface tension which is necessary to transfer a label onto a container in a manner that a label's trailing edge, containing glue, will overlap its leading edge thereby being adhesively bonded to itself rather than onto said container, after which the water naturally evaporates to leave virtually no residue.
There objects are achieved by the present invention.